Current opal assessment and evaluation practices are highly subjective as they are based on a combination of human observed factors, such as scoring the flashes of colour as the opal is moved, the body tone, colour, brightness and pattern. The fact that Australia contributes 95% of the world's opals is both a blessing and a curse. Overseas buyers often do not have the training to assess the value of opals being supplied (leading to difficult negotiations between buyer and seller), nor can they describe a particular type of opal in sufficiently concise and objective terms for the suppliers to be able to provide the appropriate opals.
Most gemstone assessment is made using a magnification 10× loupe. The gemstone is observed by holding it in a pair of gemstone tweezers and turning it (using pitch, roll and yaw) to observe light interaction with the gemstone and external and internal characteristics.
The subjective nature of current opal assessment and evaluation practices creates difficulty in negotiations and substantial distortions in the terms of trade between the opal miners and the buyers.
There exist a number of key characteristics on which an opal can be graded for its value, such as:—                Colour (hue) and area of flash        Brightness        Body tone        Pattern        Shape        Other characteristics        
In terms of relative importance to the overall gemmological value of an opal, colour and body tone combined have about a 40% weighting, followed by brightness and pattern, each at about 30%, with the remaining characteristics being of much less significant weighting. In terms of seeking an objective assessment by a human observer, colour appears to be the most difficult, followed by brightness, which seems easier to assess and quantify manually, as is the pattern, of which there are 28 main types.
Colour is difficult for a human observer to assess because of the following:                (1) The contribution of each colour changes with viewing angle, that is, with pitch, roll and yaw. All orientations are to be integrated in a colour estimation, which is difficult due to the limited perception and subjective memory of humans and their eyesight variables.        (2) Out in the field, it is difficult to accurately assign a shade according to a colour reference chart because the conditions for viewing are highly variable.        
To compound the problem, synthetic or man-made replicas of opals and other gemstones are improving, and in some cases (eg diamonds), it is extremely difficult to detect the natural gemstone from the synthetic. Verifying the authenticity of the gemstones is another problem that may be resolved by adoption of objective and automated analytical methods.
One approach to provide an objective and automated analytical method and image capture device for the grading of diamonds is disclosed in U.S. Pat. No. 6,239,867 (“the Patent”). Although also referring to use of the method and device for the grading of opals and other gemstones, that approach is unsuited to opals and is poorly suited to other gemstones, even diamonds. The image capture device and method disclosed in the Patent do not allow for gemstone movements of pitch, roll and yaw during the capture of images so as to display the “play of colours” of the gemstone, as the gemstone stage is not tiltable. Nor does the Patent disclose the assessment of all segments of the face of a gemstone for each of colour, brightness, body tone and other characteristics, many of which are especially important for opals, before grading the gemstone for each characteristic. Rather, the Patent discloses a method of sampling a small area of a diamond by moving a camera through a controlled arc and averaging the total illumination data in that area to assess the colour of the diamond.
Such a limited sampling is not suited to opals as it will not display the “play of colours”, body tone and brightness of an opal over all segments. In particular, the Patent states that gemstone colour analysis is done by obtaining average red, green and blue (RGB) values for colour images in the image pixel region delineated by the girdle and the table facets, and that by sampling the colour of a smaller region a more predictable and accurate colour reading is obtained. These procedures are not suited to the assessment of opals, where it is the entire face of the opal that requires colour assessment.
Furthermore, the image capture device disclosed in the Patent filters the light between the gemstone and its camera, and this is stated to be critical to the analysis of colours. That device also uses high viscosity immersion oil either between the gemstone and a glass plate upon which it is centrally placed or to immerse the gemstone so as to remove glint and assist in the transmission of light though the gemstone for the detection of flaws or colour inclusions. These features of the image capture device and method disclosed in the Patent are unsuited to opals and are poorly suited to other gemstones.